Rollers are widely used for materials processing including conveying, supporting and/or deforming material. In one such application, rollers are used to guide and/or support linear material as it is processed through a rolling mill such as a continuous casting and rolling mill for copper, aluminum or steel. Such guide rollers are known and used in non-ferrous rolling mills manufactured by, among others, Ashlow Engineering Ltd. of Sheffield England, Morgan Construction Company of Connecticut and Morgensheimer Company. One example of such guide rollers are rollers used in entry guides. Other rollers, including pinch rollers, 90 degree rollers for bending and changing the direction of a work piece, shaping rollers, are well known in the rolling mill art. Many rollers in such applications are made of a metallic material, often steel. Such rollers, however, are subject to a number of shortcomings. Metallic rollers are subject to large stress, pressure and abrasion in a hot and sometimes corrosive environment and thus are subject to degradation by wear, abrasion, or deformation. Such degradation can deform the product, interfere with the desired free rotation of the roller, and affect operation of other portions of the apparatus such as the bearings. Additionally, metallic rollers can contaminate the product such as by causing metallic deposits on the product.
The average useful life of a metallic roller will vary with the type and function of the roller and with the type of rolling mill. In one particular continuous copper rolling mill, for producing copper rod, it has been found that metal entry guide rollers have an average useful life of about a half week.
Other undesirable features of metal rollers are that they are typically expensive and have a relatively high density, particularly when made of steel, so that the rotating roller experiences high rotational inertia.
Rollers have also been provided which are made entirely of ceramic. However, the ceramic materials used, are relatively brittle and the rollers are subject to catastrophic failure. Analysis of such failures has provided indications that at least some failures propagate from the interior surface radially outward. It is thought that such failure may be related to stress which is caused in the ceramic when a bearing is press-fitted into the ceramic roller. It has also been found that an all-ceramic roller will sometimes fail when a new workpiece, such a new rod, is being threaded through the rolling mill. During such a threading process a rod, which is typically heated to a high temperature such as about 1100.degree. -1200.degree. F. (about 600.degree. to 650.degree. .C) strikes the roller, often with substantial force.
Other rotating devices containing ceramic are known but each has qualities which make it undesirable for use as a guide roller. U.S. Pat. No. 4,056,873 issued Nov. 8, 1977 to Cassard describes a guide roller for a rolling mill in which a ceramic annular ring is mounted on a core and held in place by first and second shoulders covering the side surfaces of the ceramic ring. The ring is placed against a first shoulder of the core and the second shoulder of the core is formed by stamping, followed by cold-boring the core to expand it and eliminate radial clearance providing the core and the ring with radial integrity. This forming process is expensive and causes stress on the ceramic ring. Moreover, it requires that non-ceramic sidewalls be provided. Such sidewalls are undesirable because providing masses larger distances from the roller rotational axis increases the rotational inertia, which may in turn require use of a softer and/or more expensive metallic material than, e.g., steel, as described in U.S. Pat. No. 4,056,873. Additionally, the side pieces require some amount of axial space, thus requiring either a thinner ceramic roller than would otherwise be possible or an increase in the space permitted for mounting the roller, often, in turn, requiring modification of the rolling mill apparatus. Indeed, the space constraints in existing rolling mills would not allow ceramic guide rollers with side plates because side plates would require that the ceramic portion be made undesirably thin and/or would require expensive modification of existing rolling mill apparatus. Additionally, provision of non-ceramic sidewalls contacting portions of the sidewalls of the ceramic part can cause stress on the ceramic due to different amounts of thermal expansion as the roller is heated.
Other configurations in which a ceramic piece is held between side pieces of another material, such as metallic material, include draw blocks and capstans such as those described in U.S. Pat. No. 4,111,026 issued Sept. 5, 1978 to Ford, et al., U.S. Pat. No. 3,621,698 issued Nov. 23, 1971 to Rocco, et al., U.S. Pat. No. 3,618,357 issued Nov. 9, 1971 to Beninga and U.S. Pat. No. 3,432,164 issued Mar. 11, 1969 to Schubert, et al. In these configurations, the metallic side pieces cause high rotational inertia, thermal/mechanical stress on the ceramic and require provision for axial thickness of the side pieces. Furthermore, these devices employ fasteners such as bolts for holding the ceramic between the side pieces, thus requiring either undesirable changes in the configuration of the ceramic pieces (such as holes therethrough or relatively thin ceramic rings) or a special configuration of the shaft or bearing as in U.S. Pat. No. 3,621,698. Furthermore, providing bolts or other fastening devices spaced from the axis of rotation of the roller further increases the rotational inertia of the roller.
Another approach to providing rotating parts involves coating a metal body with a ceramic material as described, for example, in U.S. Pats. No. 3,974,555 issued Aug. 17, 1976 to Strohmeier, et al. and U.S. Pat. No. 2,984,473 issued May 16, 1961 to Ornitz, et al. Such coated parts, however, also have a number of shortcomings. Because the coating is in integral contact with the metallic portion, cracking, lifting or other failure of the ceramic can occur because of the different amounts of thermal expansion of the ceramic and the metal. Typically, a ceramic coating is relatively thin and therefore has less longevity than a thicker ceramic part. Coating of a metallic piece is typically difficult and expensive and places constraints on the materials used because not all ceramics can be successfully coated on all metallic substrates. Additionally, because the coated part is largely metallic, the part typically has a high rotational inertia requiring more expensive bearings and/or providing an undesired amount of friction or slippage between the workpiece and the roller.
In view of the above, there is a need for a roller which is long-lasting, particularly in hot and/or corrosive environments, is relatively easy and inexpensive to make, has a relatively low rotational inertia, and can be used in existing systems such as existing rolling mill apparatus without extensive modifications thereof.